1. Field of the Invention
This invention relates to position modulated valves, and in particular, to an apparatus and method for detecting whether the friction level within the valve is approaching a condition where a malfunction of the valve is likely to occur.
2. Description of the Prior Art
As is well known, a steam turbine power plant includes within a closed-loop series arrangement a steam generator element, at least one turbine element, and a condenser element. Steam from the steam generator element is permitted to expand through the turbine, and the energy of the steam is transformed to rotational mechanical energy. The rotational energy is converted into electrical energy by a generator element operatively connected to the system. The exhausted steam from the turbine returns to the liquid state within the condenser element prior to its re-introduction into the steam generator.
Disposed at various locations throughout the power plant are flow control devices which regulate the flow of motive steam within the closed-loop power plant arrangement. Such flow control devices are usually controlled in an integrated manner by an overall power plant control system which takes into consideration, when adjusting the modulating position of each flow control device, such factors as the load imposed upon the plant, the speed at which the rotor shaft is turning and various other system parameters.
One of the system flow control devices is provided intermediate between the steam generator element and the inlet of the turbine apparatus. This flow control device, commonly a position-modulating valve, when provided in such a location is known in the art as the control valve for the turbine. By varying the location of the valve plug relative to the valve seat disposed within the casing of the valve, the volume of motive steam able to pass from the valve inlet (connected to the steam generator) and to the outlet port thereof (and into the turbine element) is precisely regulated. Varying the dimension of the plug away from its associated seat, known as the "lift", varies the volume of motive steam passing into the turbine element. If, for example, during low load conditions imposed upon the power plant as a whole, the flow control device narrows the lift to reduce the volume of motive steam passing into the turbine. Conversely, during periods of peak power demand on the plant, the lift is widened so as to permit greater volumes of motive steam to enter the turbine.
The most common failure of such a position modulating valve, disposed either as the turbine control valve or at any other location within the power plant, is that such flow control devices tend to stick. Such a failure mode is normally attributable to deposits accreting on the valve stems where there exist narrow clearances between that stem or a moving part associated therewith and the valve casing. Of course, if the deposits of clogging elements accumulate to a sufficient magnitude, the control valve plug is unable to move, and its position modulating function is totally vitiated.
Incipient failures of this type may be normally detected during valve test cycle which is generally performed at least once a week in most power generating stations. However, valve testing as it is carried out in the prior art determines only whether the valve plug is free to move. Such testing does not predict if the valve is close to being stuck-- that is, if the accumulation of deposits within the narrow clearance has not yet reached the point where the valve is completely stuck, but is accumulating within those clearances so that the point of sticking is near. During such inspections, of course, if a valve is found to be stuck, corrective action is taken immediately and forced outages usually result. However, if the valve has moved during the system test, no further action is taken, even though the accumulated deposits within the narrow clearances within the valve may be precipitously close to causing a stickage within a valve.
It is, of course, desirable to provide an apparatus and method for determining whether the friction level within a position modulated flow control valve is below a certain predetermined friction level. Such information is most useful in determining potential trouble spots before the valves become totally stuck; and, when such testing apparatus is associated within the overall computer-controlled valve control system, the information provided thereby is invaluable to the prevention of forced outages and the loss of power generation capability.